This disclosure generally relates to a method for operating an isolation valve. In particular, this disclosure relates to a method of operating a fuel tank isolation valve for controlling fuel vapor flow between a fuel tank and a fuel vapor collection canister.
It is believed that prior to legislation requiring vehicles to store hydrocarbon vapors that are generated when refueling a vehicle, a simple orifice structure was used to maintain a positive pressure in a fuel tank to retard vapor generation. It is believed that such orifice structures could no longer be used with the advent of requirements controlling on-board refueling. It is believed that, on some vehicles, the orifice structure was simply deleted, and on other vehicles, the orifice structure was replaced with a diaphragm-actuated pressure relief valve. It is believed that these diaphragm-actuated valves suffer from a number of disadvantages including that the calibration (i.e., pressure blow-off level) changes with temperature and age.
It is believed that it is necessary on some vehicles to maintain an elevated pressure in the fuel tank to suppress the rate of fuel vapor generation and to minimize hydrocarbon emissions to the atmosphere. It is believed that under hot ambient temperature conditions or when the fuel is agitated, e.g., when a vehicle is operated on a bumpy road, the amount of fuel vapor generated can exceed the amount of fuel vapor that can be purged by the engine. It is believed that a carbon canister can become hydrocarbon saturated if these conditions occur and are maintained for an extended period. It is believed that such a hydrocarbon saturated carbon canister is unable to absorb the additional fuel vapors that occur during vehicle refueling, and that hydrocarbon vapors are released into the atmosphere. A legislated standard has been set for the permissible level of free hydrocarbons that may be released. A so-called xe2x80x9cshed testxe2x80x9d is used to measure the emission of the free hydrocarbons for determining compliance with the legislated standard.
It is believed that there is a need to provide a method for operating a fuel tank isolation valve that overcomes the drawbacks of orifice structures and diaphragm-actuated pressure relief valves. Moreover, it is believed that there is a need to provide a method for operating a fuel tank isolation valve that maintains an elevated pressure in a fuel tank and isolates the fuel tank from direct purging.
The present invention provides a method of operating a fuel tank isolation valve. The fuel tank isolation valve has a first port, a second port, an electric actuator, and a valve body. The first port is in fluid communication with a fuel vapor collection canister. The second port is in fluid communication with a fuel tank. And the electric actuator moves the valve body to control fluid communication between the first and second ports. The method includes supplying a first electric signal to the electric actuator such that the valve body permits substantially unrestricted fuel vapor flow between the first and second ports, supplying a second electric signal to the electric actuator such that the valve body substantially prevents fuel vapor flow between the first and second ports, and supplying a third electric signal to the electric actuator such that the valve body provides restricted fuel vapor flow between the first and second ports.